Waste water treatment plants use chlorine for controlling various bacteria in water effluents. However, concern as to toxicity of chlorine and chlorine-containing chemicals in the effluent to aquatic organisms has resulted in pressure on operators of waste water treatment plants to reduce residual chlorine in waste water effluents discharged into natural waters.
The amount of residual chlorine permitted in water effluents varies from one facility to another. Normally the allowable amount of chlorine is typically less than about 100 micrograms per liter as "total" chlorine. Total chlorine is the terminology used to express chlorine which may exist in the free available form plus that which is combined with nitrogen compounds (chloramines). At some facilities the total chlorine level in discharged water must be less than 5 micrograms per liter. The amount of chlorine in the treated effluent must be monitored frequently by the waste water facility.
Historically, wastewater facilities have used probe detectors, such as those based on amperometric, potentiometric, or polarographic principles to analyze low chlorine levels present in their treated wastewater. One serious problem with these devices has been the fouling of the metal electrode or membrane surfaces that are in contact with the wastewater sample. The fouling can cause a drift of the electrode output or a diminished response which will result in errors in the measurement. These probe devices also require special operator skills to achieve accurate results below 50 micrograms per liter chlorine in wastewater analyses.
Colorimetric analytical methods for chlorine, such as those published in Standard Methods for the Examination of Water and Wastewaters, have only limited application for measuring trace chlorine levels in wastewaters. The colorimetric methods are limited by instability of the reagents formulated for the test, instability of the colored reaction product, and inadequate compensation for turbidity, color and particulate matter. In addition, the reagents added to the sample can cause a "reagent blank" due to impurities in the reagents, which may lead to serious errors at the trace measurement levels if not compensated for accurately.
There has not heretofore been provided an effective ultra-low range test for chlorine determination in wastewater at levels below 50 .mu.g/liter.